The prior art has provided numerous methods for manufacturing porcelainized metal substrates. These methods comprise spraying a porcelainized mixture upon the metal substrate, drying the mixture and then firing the combination; utilizing electrolytic deposition processes whereby the metal piece is placed within a tank containing a porcelainized mixture and electric current utilized to deposit the mixture onto the surface of the substrate; or conventional screening methods wherein a porcelain mixture is screen printed onto a metal substrate and followed by drying and firing. These prior art processes have proved to be time consuming, expensive and do not readily lend themselves the multilayering of the porcelain mixtures. Thus, there is a need to develop a method for providing a facile manner of handling the porcelain mixture, so that it may be more easily applied to a metal substrate without the use of the time consuming and expensive prior art methods. Also, there is a need for a simplified method of providing for multilayering of porcelain layers whereby selected porcelain mixture layers may support thick film electrical circuits therebetween and the porcelain layers, thick film circuits, and metal substrate may be all fired during a single firing step.
Several prior art methods utilizing ceramic or glass frit tapes have been developed. Doolittle et al. U.S. Pat. No. 3,293,072 issued Dec. 20, 1966 and entitled "Ceramic-Metallizing Tape" describes a method for providing a self-supporting metallizing film as a sealing tape which may be applied to a ceramic surface for the purpose of sealing the surface to the surface of a metal or ceramic element. Ettre U.S. Pat. No. 3,371,001 issued Feb. 27, 1968 and entitled "Method of Applying Uniform Thickness of Frit on Semi-Conductor Wafers" describes the use of a very thin glass layer as a passivation encapsulation, protective and dielectric layer formulation on a semi-conductor wafer. Ettre U.S. Pat. No. 3,506,473 issued Apr. 14, 1970 and entitled "Method of Transferring Glass Frit Image From Transfer Sheet" describes a multilayer transfer tape for transferring a thin glaze, glass or ceramic layer as an insulating layer to a refractory or metal substrate by utilizing only pressure to adhere the tape to the substrate, and pulling away the unadhered sections to form the patterns to be fired. Bennet et al. U.S. Pat. No. 3,518,756 issued July 7, 1970 and entitled "Fabrication of Multilevel Ceramic, Microelectronic Structures" describes the manufacture of multilayer ceramic microelectronic structures having metallized patterns and metallized through holes, the ceramic layers being punched and pressed together before firing. The above patents all deal with the formulation of ceramic or glass tapes for effecting the respective objects specified in the issued patents, but none of the patents describe a process suitable for utilizing a flexible porcelain tape to provide a porcelainized metal substrate, particularly a porcelainized metal substrate suitable for supporting a thick film electrical circuit thereon.